Wind energy is currently a fast growing power generation technology, and ongoing development is directed to providing wind-generated power to electrical power grids. Power conversion systems are needed to adapt the power generated by the wind turbines to AC electric power in a form compatible with the power grid. One type of conversion apparatus is a current source converter (CSC) that includes a current source rectifier (CSR) and a current source inverter (CSI).
As WECSs become more prevalent, utility operators must ensure the reliability and efficiency of the power system, including compliance with grid connection codes applicable to distributed generators including wind power generators. One such requirement is the capability of WECSs to ride-through grid fault conditions to prevent disconnection of a large wind generator caused by network disturbances to avoid or mitigate system instability and generator trips. Other typical requirements such as reactive and active power regulation based on the system voltage and frequency are also specified. Compliance with these requirements impacts the design of power converters and controllers for WECSs. Currently, the most prevalent WECS configuration is a variable-speed wind turbine used with either a doubly fed induction generator (DFIG) with a partial-rated power converter or an induction/synchronous machine equipped with a fully-rated power converter. DFIG configurations are popular because of the reduced size of the converter (about ⅓ to ¼ of the total KVA rating), but the fault ride-through capability of DFIG systems is limited and additional hardware is required in most cases. Direct-drive permanent magnet synchronous generator (PMSG) solutions with a full power converter are an attractive alternative as these are completely decoupled from the power grid, and provide wide operating range with fault-ride-through capability. In addition, the provision of a permanent magnet rotor (without electrically excited rotor windings) improves system efficiency and eliminates the need for slip-ring and maintenance, making the PMSG solution ideal for high power offshore applications.
Most conventional drive system control schemes assume static grid behavior and are thus not well adapted for accommodating grid fault conditions. A short circuit grid fault and the resultant converter terminal voltage drop may cause the grid side converter to lose its control capability. Unbalanced power flow at the input and output during transients can cause over-current or over-voltage in the converters and trigger the system protection and ultimately converter shut down. Previous fault ride-through techniques have largely been focused on voltage source converters (VSC) in WECS, such as electronic dynamic braking to dump the excessive energy to external resistors or energy storage systems, or allowing the incoming wind energy to be temporarily stored in the moment of inertia of the turbine-generation system. Other proposed fault-condition techniques employ nonlinear control methods to improve the conventional current control performance, but the implementations are complex and very sensitive to system variables. Pulse width modulated (PWM) current source converter (CSC) topologies, compared with VSC based configurations, provide a simple topology solution and excellent grid integration performance such as sinusoidal current and fully controlled power factor, where a DC link reactor provides natural protection against converter short circuit faults. However, unlike VSCs, the grid voltage fault ride through of a CSC-based WECS has been rarely studied in the literature. Accordingly, there is a need for improved wind energy systems by which energy derived from wind-driven machines can be converted in a CSC for supplying electrical power to a grid with the capability of riding through grid fault conditions without introduction of additional hardware.